The invention relates to transmitter apparatus for an information transmission system, which system comprises at least two such apparatuses having their outputs coupled to a common transmission path; said apparatus comprising a transmitter, an indicator circuit, constructed to generate an output signal indicative of whether or not the transmission path is in use, and a control circuit having an input for a transmit-request signal, the output of said indicator circuit being coupled to said control circuit and said control circuit being constructed to generate at an output thereof, which output is coupled to a control input of said transmitter, a transmission activation signal for said transmitter in response to the application of a transmit-request signal to the control circuit input, the instant of generation of said activation signal depending on the output of said indicator circuit in such manner that, if said indicator circuit is indicating that the transmission path is in use when the transmit-request signal is applied to the control circuit input, said activation signal will be generated at an instant which occurs a predetermined time interval after the indicator circuit subsequently indicates that the transmission path has become clear, and only then provided that the indicator circuit is then indicating that the transmission path is clear.
U.S. Pat. No. 2731635 discloses an information transmission system comprising apparatuses of this type in which, if transmit-request signals are applied to the inputs of the control circuits of a plurality of apparatuses of the system while another apparatus of the system is transmitting and hence occupying the transmission path, the apparatuses of the plurality will be put into a hold condition until the transmission has ended. At the instant that the transmission ends the indicator circuits of the apparatuses of the plurality indicate this fact and the corresponding control circuits activate the corresponding transmitters to transmit intelligence at instants which each occur a predetermined time interval thereafter, provided that the corresponding indicator circuits are indicating that the transmission path is clear at said instants. The predetermined time intervals are fixed and a different one is assigned to each apparatus. Thus, after the transmission path becomes free, that apparatus in the hold condition to which the shortest predetermined time interval has been assigned will commence transmitting its intelligence first, resetting the other apparatuses which were in the hold condition, back to the hold condition until the transmission path again becomes free. In this way the apparatuses of the system are given a predetermined and fixed order of priority for transmission if more than one should attempt transmission at the same time.
It is sometimes desirable that a "first-come first-served" order of priority be established in an information transmission system in which several apparatuses of the system, for example several mobile radios, are coupled to a common transmission path, for example to a broadcast channel or group of broadcast channels to a base station, if more than one such apparatus attempts to transmit while the transmission path is in use. Of course such an order of priority could be established by providing a further broadcast channel to the base station, over which each apparatus transmits a request signal when it requires to transmit, the base station then keeping an account of the order in which the various requests are issued and transmitting, each time the path ceases to be in use, a "request-accepted" signal to the apparatus which is at the head of the queue at that time. However, the provision of the further channel is wasteful in that it will occupy valuable space in the (limited) part of the broadcast spectrum which is available. It is an object of the invention to mitigate this disadvantage.